The present invention is directed to hydraulic lash adjusters used in the valve trains of internal combustion engines and the like and more particularly to hydraulic lash adjusters of the self-contained type in which the fluid is sealed within the assembly with no provision for its replenishment from any external source during operation.
In the valve trains of internal engines a pre-determined valve clearance is generally provided in order to compensate for thermal expansion of various parts in the valve train and for frictional wear of the parts of the valve train mechanism. In order to keep the valve clearance in the zero state during the valve operation to thereby stabilize the valve operation, the self-contained hydraulic lash adjusters are slidably positioned between a cam upon the cam shaft and a push rod. Selfcontained hydraulic lash adjusters are generally comprised of a cup-shaped cylindrical body which is loosely fitted within a bore in the engine block. The body is provided with an upwardly open bore in which a plunger is slidably received to thereby define an oil reservoir and an oil pressure chamber within the body. The bottom wall of the plunger is provided with an oil passage having a one-way ball-type check valve associated therewith to allow only flow of fluid from the reservoir within the plunger to the pressure chamber which is located between the bottom of the plunger and the bottom of the body. Leakage clearance is provided between the body and the plunger and passages are provided in the side-wall of the plunger to provide for the return of oil from the pressure chamber to the reservoir when the fluid in the pressure chamber is pressurized. In some self-contained hydraulic lash adjusters, a flexible bag-shaped diaphragm of silicon rubber or the like is secured within the bore of the plunger to define the surface of the reservoir. The interior of the diaphragm is maintained in communication with the atmosphere by means of an air passage provided through a cap-like member located in the upper end of the bore of the body in contact with the upper end of the plunger.
Examples of such prior art lash adjusters are disclosed in the U.S. Pat. Nos. to Kodama, 4,191,142, granted Mar. 4, 1980; and 4,338,894, granted July 13, 1982, both of which are assigned to the assignee of the present application.
In the operation of such prior art lash adjusters the oil within the pressure chamber is returned to the reservoir chamber by means of the leakage clearance and oil passages in the side wall of the plunger when the valve is lifted while the oil within the reservoir is transmitted to the pressure chamber through the oil passage and one-way check valve located in the bottom of the plunger when the valve is returned. Thus, the volume of oil within the reservoir chamber repeatedly increases and decreases. In order to smoothly vary the oil volume, the diaphragm also repeatedly expands and contracts since the interior of the bag-like diaphragm is in communication with the atmosphere.
If the valve which is associated with such a lash adjuster is stopped in its lifted position when the engine is stopped, the lash adjuster will be subject to the biasing force of a spring through the rocker arm and push rod. Therefore, the plunger in the lash adjuster is forced downwardly and the oil within the pressure chamber continues to flow into the reservoir through the leakage clearance and the oil return passages. As a result, the diaphragm is gradually compressed to its completely compressed condition. Under these conditions it is possible that the lowermost end of the diaphragm will contact the bottom wall of the plunger surrounding the oil passage between the reservoir chamber and the pressure chamber and thus prevent the flow of oil from the reservoir chamber to the pressure chamber upon renewed operation of the engine. As a result, the self-contained lash adjuster will be inoperative. While such a problem can be avoided by increasing the distance between the lower end of the diaphragm in the totally compressed condition and the bottom wall of the plunger, it is preferable that the distance be reduced as small as possible in order to minimize the inertial mass of the lash adjuster to thereby improve the following movement of the valve with respect to the movement of the cam. Furthermore, it is desirable to have the overall axial range of the lash adjuster as small as possible to assist in the miniaturization of the engine per se.